8-Layer 6.6mil Rogers RO4350B + Isola I-Tera MT40 Hybrid PCB 1.636mm Pressed Thickness ENEPIG

Part I — Product Description

1. Product Overview

This 8-layer hybrid printed circuit board is built for RF and microwave products that require high-frequency signal integrity on both outer faces while retaining a high-speed, thermally robust digital body. Each outer sub-stack is built from a 6.6 mil (0.168 mm) Rogers RO4350B hydrocarbon-ceramic core—layers 1–2 on the top and layers 7–8 on the bottom—where the controlled-impedance RF lines, antenna feeds and matching networks are routed. The central body is constructed fromIsola I-Tera MT40 very-low-loss cores and prepreg, an FR-4-process-compatible dielectric system that provides the thermal margin and reliability required for lead-free assembly and sequential lamination.

The panel is pressed to a nominal 1.636 mm on a symmetric build, with 1 oz copper on the two outer layers and 0.5 oz copper on the six inner layers. Interconnects combine L1–L2 and L7–L8 blind vias in the two RO4350B sub-stacks with a central through-hole, while a top-side 6 mil single-ended line is controlled to 50 Ω. The board is finished with electroless nickel/electroless palladium/immersion gold (ENEPIG), green solder mask and white legend, and every panel is 100% electrical tested against the net list before shipment.

2. Product Highlights

· Dual RF faces: 6.6 mil (0.168 mm) RO4350B on both outer sub-stacks, with process Dk 3.48 and Df 0.0037 at 10 GHz, placing controlled low-loss RF geometry on the top and bottom layers.

· Very-low-loss body: I-Tera MT40 cores and prepreg with Dk 3.45, Df 0.0031, Tg 215 °C and Td 360 °C, FR-4 process compatible and CAF resistant.

· Symmetric 1.636 mm build: two 0.168 mm RO4350B cores, two 0.381 mm MT40 cores and three 0.12 mm MT40 prepreg bonds, balanced about the center.

· Advanced interconnects: L1–L2 and L7–L8 blind vias formed by sequential lamination, plus a central through-hole.

· Controlled impedance: a 6 mil single-ended line on L1 (TOP) held at 50 Ω; 1 oz outer copper and 0.5 oz inner copper.

· ENEPIG surface finish, green solder mask with white legend, and 100% net-list electrical test on every board.

3. PCB Construction Details

Table 1 summarizes the construction, materials, finish and quality provisions of the 8-layer symmetric hybrid board as manufactured.

Table 1. PCB Construction Details

4. PCB Stack-Up

Table 2 details the layer-by-layer dielectric and copper build from the top face to the bottom face; the values sum to the nominal 1.636 mm pressed thickness illustrated in Figure 1.

Figure 1. 8-layer RO4350B + I-Tera MT40 symmetric hybrid stack-up, with L1–L2 and L7–L8 blind vias and a central through-hole; nominal pressed thickness 1.636 mm.

5. Engineering Argument 1 — Dual RO4350B RF Sub-Stacks on an MT40 Body

5.1 RO4350B carries RF on both outer faces

Rogers 4350B is a woven-glass-reinforced hydrocarbon/ceramic thermoset laminate engineered for near-PTFE high-frequency performance but processed on standard FR-4 lines. Its process dielectric constant is 3.48 ± 0.05 at 10 GHz and its dissipation factor is 0.0037 at 10 GHz, roughly an order of magnitude below general-purpose FR-4; the dielectric constant stays broadly stable over temperature and across frequency, and the glass transition temperature is above 280 °C. Placing a6.6 mil RO4350B coreon the top and again on the bottom means both outer faces carry controlled, low-loss RF geometry—the key difference from a single-sided RF hybrid, because a module can place antennas, matching networks or front-end copper on both sides with a consistent dielectric reference. As a thermoset rather than PTFE, RO4350B needs no sodium-etch through-hole preparation, so the RF sub-stacks can be incorporated into a conventional multilayer flow.

5.2 I-Tera MT40 carries the high-speed body

The I-Tera MT40 central body performs a different role. MT40 is a very-low-loss, non-PTFE laminate and prepreg system with Dk 3.45 and Df 0.0031 at 2 GHz, with Isola reporting the Dk stable between −55 and +125 °C up to W-band frequencies. Its Tg of 215 °C (DSC) and Td of 360 °C provide the thermal headroom for lead-free reflow and multiple lamination cycles, while documented CAF resistance and FR-4 process compatibility suit dense, high-reliability multilayer construction. Using MT40 for the inner high-speed, control and power layers—rather than carrying the entire stack in RF material—removes the cost premium of an all-Rogers build without sacrificing electrical performance on the RF layers. Because MT40 is supplied as both laminate and prepreg, it is able to form the inner cores and the three bonding layers from one matched material system.

6. Engineering Argument 2 — Holding the 1.636 mm Symmetric Build

6.1 Thickness budget across four cores

At 1.636 mm the build is fully specified layer by layer, and the thickness budget closes exactly. The two 0.168 mm RO4350B cores contribute 0.336 mm, the two 0.381 mm MT40 cores contribute 0.762 mm, the three 0.12 mm MT40 prepreg bonds contribute 0.360 mm after lamination, and the eight copper layers—1 oz (0.035 mm) on the outside and 0.5 oz (0.018 mm) inside—contribute 0.178 mm, for a total of 1.636 mm. Because prepreg resin flow shifts the final bond thickness, the press program uses controlled glass styles and resin content together with measured core thicknesses to hold the nominal, which in turn protects the dielectric height that determines the 50 Ω geometry.

6.2 Symmetry, CTE matching and sequential lamination

The construction is symmetric about its center: an MT40 core and prepreg bond above the midplane is mirrored below, and the two RO4350B outer cores are equal in thickness and copper weight. Symmetry is the primary control for bow and twist, because it balances resin shrinkage and the differing z-axis expansion of the two resin systems as the panel cools. RO4350B expands at 10/12/32 ppm/°C in the X/Y/Z axes, while MT40 lists 12 ppm/°C in-plane and 55 ppm/°C below Tg through the z-axis; the closely matched in-plane values and a staged, FR-4-style ramp-and-soak profile allow the two materials to coexist without locked-in residual stress. The two blind-via sub-stacks require sequential lamination, for which MT40 is explicitly qualified through multiple lamination cycles, and the press follows MT40's roughly 200 °C cure profile.

6.3 Controlled 50 Ω geometry

Impedance control is implemented on the top RO4350B face, where the 6.6 mil (0.168 mm) dielectric height sets the 50 Ω single-ended microstrip geometry at a 6 mil trace width. The tight Dk tolerance of RO4350B, the controlled outer copper weight and the specified prepreg thickness together keep the fabricated trace impedance repeatable across the panel and between production lots. The same dielectric reference is mirrored on the bottom sub-stack, allowing additional controlled RF lines to be added on L8 if the design requires it.

7. Interconnect and Surface Finish

The blind vias are deliberately placed within the two RO4350B sub-stacks. In RF layouts, a short blind via from the outer RF layer to its adjacent reference plane gives the lowest-inductance return path and avoids the long via stub that a through-hole would leave through an 8-layer board—a measurable benefit at the frequencies where RO4350B operates, while the central through-hole handles conventional interlayer connection. ENEPIG deposits nickel, a thin palladium layer and immersion gold over the copper. The palladium barrier prevents the nickel corrosion (the “black pad” risk) associated with ENIG, while the gold protects the surface during storage and enables fine-pitch SMT, multiple reflow cycles and, where required, aluminum wire bonding—making it a robust finish for high-reliability RF and automotive assemblies. Controlled impedance is verified on production coupons by time-domain reflectometry, and blind-via formation, through-hole plating and the RO4350B–MT40 bond are confirmed by cross-section before the 100% net-list electrical test is applied.

8. Typical Applications

Combining dual-face RO4350B RF performance with the very-low-loss, lead-free I-Tera MT40 body, this symmetric hybrid build is suited to:

· 5G active antenna systems and massive-MIMO front ends carrying RF on both faces

· Microwave point-to-point links and cellular base-station power amplifiers

· Automotive radar and sensor modules

· Satellite and GNSS antennas, LNBs and antenna feeds

· RF/microwave transceivers and matching networks combining analog RF with high-speed digital control

· Instrumentation and phased-array modules requiring controlled impedance and high component density

9. Conclusion

This 8-layer RO4350B + I-Tera MT40 hybrid PCB is an engineered balance rather than a compromise. It places 6.6 mil RO4350B on both outer faces for low, stable, tightly controlled RF performance, uses very-low-loss MT40 in the central body for high-speed signals, thermal margin and FR-4 manufacturability, and locks the result into a symmetric 1.636 mm build with L1–L2 and L7–L8 blind vias, a controlled 50 Ω line and ENEPIG. For RF and microwave modules that need high-frequency performance on both sides without the cost and processing overhead of an all-Rogers or all-PTFE stack, the construction is a repeatable, production-qualified answer rather than a prototype-only design.

Part II — CCL Material Knowledge Guide: Rogers RO4350B and Isola I-Tera MT40

A copper-clad laminate (CCL) is the dielectric-and-copper base from which a PCB is patterned. In ahybrid board, two CCL systems with different resin chemistry share one pressed panel. The board in Part I combines Rogers RO4350B on the outer RF sub-stacks with Isola I-Tera MT40 in the central body and in the bonding layers. The guide below presents the two materials independently, using manufacturer typical-value data and charts; each section can be read on its own.

Figure 2. Copper-clad laminate cores and patterned high-frequency panels; the dielectric-and-copper base from which hybrid PCBs are built.

1. Rogers RO4350B High-Frequency Laminate

RO4350B is a member of the Rogers RO4000 series, a family of woven-glass-reinforced hydrocarbon/ceramic thermoset laminates designed to deliver electrical performance close to PTFE/woven-glass while using the same fabrication methods as standard epoxy/glass. The result is a low-loss, tight-dielectric-tolerance material that runs on FR-4 equipment at a fraction of the cost of conventional microwave laminates. Unlike PTFE-based materials, RO4350B does not require specialized through-hole preparation such as sodium etch, and it is rigid enough for automated handling and standard copper-surface scrubbing. RO4350B is the RoHS-compliant, UL 94 V-0 flame-retardant member of the RO4000 family and is used as a drop-in replacement for RO4003C where V-0 certification is required.

Two dielectric-constant figures are commonly quoted. The process Dk of 3.48 ± 0.05 is measured at 10 GHz and 23 °C by clamped stripline; the design Dk of 3.66 is an average value from 8–40 GHz differential-phase-length testing and is the recommended starting point for field-solver work, while the 4 mil core has a separate process Dk of 3.33 ± 0.05. Figures 3 and 4 show the characteristic stability of the RO4000 family versus temperature and frequency, and Figure 5 shows measured microstrip insertion loss—the curve set includes a 6.6 mil RO4350B build, the very dielectric thickness used on the outer sub-stacks of the board in Part I.

Figure 3. RO4000 series dielectric constant versus temperature; RO4350B maintains a comparatively flat response across the operating range. Source: Rogers RO4000 series data sheet.

Figure 4. Dielectric constant versus frequency by differential phase-length method, including the RO4350B curve. Source: Rogers RO4000 series data sheet.

Figure 5. Microstrip insertion loss by differential length method; the 6.6 mil RO4350B curve (center) demonstrates the loss advantage of the dielectric used on the outer sub-stacks. Source: Rogers RO4000 series data sheet.

Table 3. Rogers RO4350B Typical Properties (Manufacturer Data Sheet Set)

RO4350B is offered in standard thicknesses from 0.0066 in to 0.060 in, on panel sizes up to 48 × 36 in, with electrodeposited 1/2 oz (18 µm) and 1 oz (35 µm) copper; reverse-treated and LoPro foil are available for lower surface roughness and reduced insertion loss.

2. Isola I-Tera MT40 Very Low-Loss Laminate and Prepreg

I-Tera MT40 is Isola's very-low-loss laminate and prepreg system for high-speed digital and RF/microwave printed circuit designs. Its dielectric constant is 3.45 and its dissipation factor is 0.0031 at 2 GHz, and Isola reports the Dk to remain stable between −55 and +125 °C up to W-band frequencies, which keeps high-speed channels and RF response predictable over temperature. The low Df makes MT40 a cost-effective alternative to PTFE and other microwave laminates, and—crucially for a hybrid multilayer—it does not require the special through-hole treatments commonly needed for PTFE-based cores.

Thermally, MT40 provides a Tg of 215 °C by DSC (230 °C by DMA), a Td of 360 °C at 5% weight loss, a T260 time-to-delamination greater than 60 minutes and total z-axis expansion of 2.8% from 50 to 260 °C. It is RoHS compliant, lead-free assembly compatible, multiple-reflow and multiple-lamination-cycle capable, CAF resistant and fully FR-4 process compatible. The multiple-lamination-cycle qualification is directly relevant to the board in Part I, whose two blind-via sub-stacks are built by sequential lamination. Isola recommends a roughly 200 °C cure (product temperature not exceeding 210 °C), a controlled 3–5 °C/min heat ramp and staged vacuum and pressure, with prepreg stored at or below 23 °C and under 50% relative humidity.

Table 4. Isola I-Tera MT40 Typical Properties (Complete Manufacturer Data Sheet Set)

MT40 is offered in both laminate and prepreg form—laminate from 2 to 24 mil (0.05 to 0.61 mm)—providing a complete, matched material solution for multilayer, hybrid, RF/microwave and double-sided designs. Copper foil options include HVLP (VLP2, ≤2.5 µm Rz), reverse-treated foil and embedded-resistor foil in 1/2, 1 and 2 oz (18, 35 and 70 µm), with E-glass, square-weave and mechanically spread glass fabrics; HVLP foil is particularly useful for minimizing conductor-surface-roughness loss in high-speed channels. The system is manufactured in Asia, North America and Europe, and finished boards are shipped in moisture-barrier packaging with a humidity indicator for lead-free assembly.

3. How the Two CCLs Complement Each Other

Table 5 places the key properties side by side. Both materials are non-PTFE and run on FR-4 processes, which is the practical reason they can be combined in one press, but they occupy different points on the performance and cost map.

Table 5. RO4350B versus I-Tera MT40 — Key CCL Property Comparison

The two materials are not competitors in this design. RO4350B brings the lowest outer-layer loss and an established high-frequency pedigree, while MT40 supplies a very low Df, a complete matched prepreg system, high thermal margin and lower unit cost for the inner layers. Their in-plane CTE values are close (10/12 ppm/°C for RO4350B and 12 ppm/°C for MT40), and the fact that both are non-PTFE, FR-4-process systems is what makes the hybrid manufacturable rather than experimental.

4. CCL Conclusion

Selecting a CCL defines the electrical and thermal envelope of the finished board. RO4350B brings a 3.48 process Dk, a 0.0037 dissipation factor, UL 94 V-0 and a proven high-frequency pedigree to the outer RF faces, while I-Tera MT40 brings a 0.0031 Df, W-band Dk stability, a 215 °C Tg and a fully matched laminate-and-prepreg system to the high-speed body. For designers, the rule is consistent with the construction in Part I: specify the proven RF material on the layers whose signal budget demands it, and let a very-low-loss, FR-4-process material carry the rest. The hybrid then delivers both the measured high-frequency performance and the unit economics that a volume RF product requires.